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Air New Zealand says it has saved the enough fuel for 20 flights between Auckland and Los Angeles by plugging in to mains power while on the ground here during the past year.

Instead of using big Auxiliary Power Units in their tails, jets are running off mains power to run onboard lighting, air conditioning and mechanical systems at the gate when the main engines are off.

The airline estimates that during the past year it has saved 2.5 million litres of fuel using electricity rather than jet fuel to power its Boeing 777, 787-9 and Airbus A320 aircraft at departure gates at Auckland and Christchurch International Airports.

This collaboration with airports has cut the airline's annual APU use by a third since July 1 last year, saving more than 6 million kilograms of carbon – the equivalent of removing 1000 cars from the road.

Air New Zealand is the country's biggest single user of petroleum products and last year its carbon footprint grew by 5.1 per cent, lower than its 6.3 per cent capacity growth. This was mainly due to more efficient aircraft joining the fleet which Morgan said were where the step change in reducing its carbon impact could be made.

In its latest sustainability report it said an average annual fuel efficiency improvement of 1.5 per cent was equal to 49,000 tonnes of carbon, a greenhouse gas.

Air New Zealand chief operational Integrity and standards officer David Morgan said the switch to ground power for its international aircraft was just one measure the airline has in place to curb fuel use and minimise carbon impact. The forecast savings had exceeded expectations.

"While Air New Zealand's fleet is one of the most modern and fuel efficient globally, we must keep pushing ourselves to operate smarter – both on the ground, and in the air, through initiatives such as flight path and climb optimisation, and reducing weight on board.

Ground power is used to keep aircraft running on the ground at some airports. Photo / Supplied

The airline used on average about 150,000 litres of fuel an hour and that represented 99.5 percent of the airline's total carbon impact.

''Hunting down efficiencies is the top priority for our carbon reduction programme and it's fantastic to see new ground processes at Auckland and Christchurch achieve these significant savings.''

Morgan said that during the Marsden Point pipeline crisis last year, where airlines operating out of Auckland were restricted to just 30 percent of standard fuel use, the switch to ground power saved enough fuel to operate 40 more domestic flights than it would have been able to otherwise.

The airline had worked closely with both Auckland and Christchurch International Airports to ensure ground infrastructure was compatible with aircraft systems and processes are aligned, and is now training staff to adopt the new processes at Wellington Airport.

He said that in the coming year Air New Zealand will explore ground power at its offshore ports as well as the savings potential of single-engine taxiing to the gate, which could save 20 litres of fuel per minute on the Boeing 787-9 aircraft. The airline was working with Boeing to establish protocols for safe taxiing around airfields on one engine.

The auxiliary power unit is usually run on the ground during passenger boarding and when they get off the aircraft.

The APU turns an electric generator that powers the electrical system on the aircraft when the main engines are off. It also provides pneumatic pressure for air conditioning and can start the main engines. Morgan said Air New Zealand still uses the APU to do this as that would mean staying at the gate to start the engine.

Air New Zealand has signed up to the aviation industry's mid- and longer-term goals of carbon neutral growth from 2020, and halving 2005 emissions levels by 2050.